Field
The present invention relates to measuring devices and especially to a gyroscope structure, as defined in the preamble of independent claim 1, and to a method for manufacturing a gyroscope structure, as defined in the preamble of independent claim 11. The present invention further relates to an improved gyroscope structure and more particularly to a structure enabling quadrature error compensation in a gyroscope, and to an improved method for manufacturing a gyroscope structure enabling use of mechanical quadrature error compensation in a gyroscope.
Description of the Related Art
Micro-Electro-Mechanical Systems, or MEMS can be defined as miniaturized mechanical and electro-mechanical systems where at least some elements have a mechanical functionality. Since MEMS devices are created with the same or similar tools used to create integrated circuits, micromachines and microelectronics can even be fabricated on the same piece of silicon.
MEMS structures can be applied to quickly and accurately detect very small changes in physical properties. For example, a microelectronic gyroscope can be applied to quickly and accurately detect very small angular displacements.
Motion has six degrees of freedom: translations in three orthogonal directions and rotation around three orthogonal axes. The latter can be measured by an angular rate sensor, also known as gyroscope. In MEMS gyroscopes, Coriolis Effect is used to measure angular rate. When a mass is moving in one direction called the primary motion and rotational angular velocity is applied, the mass experiences a force in orthogonal direction as a result of the Coriolis force. Resulting physical displacement caused by the Coriolis force may be then read from, for example, a capacitive, piezoelectrical or piezoresistive sensing structure. The displacement due to the Coriolis Effect may also be called sense mode. The primary motion may alternatively be called primary mode or drive mode.
In MEMS gyroscope, mechanical oscillation is used as the primary motion. When an oscillating gyroscope is subjected to an angular motion orthogonal to the direction of the primary motion, an undulating Coriolis force results. This creates a secondary oscillation, also referred to as the sense mode, orthogonal to the primary motion and to the axis of the angular motion, and at the frequency of the primary oscillation. The amplitude of this coupled oscillation can be used as the measure of the angular rate, i.e. the absolute value of angular velocity.
In an ideal gyroscope, the drive mode movement and the sense mode movement are perfectly orthogonal, so that without the Coriolis force, the sense mode is not excited. However, in practical implementation of a MEMS gyroscope, the device is not necessarily quite optimal or symmetric. In many cases, this is caused by the un-ideal features in the manufacturing process. In example, some etched walls may not be perfectly vertical but may become angled, or material layer thickness may vary slightly. Then, the drive mode may not be perfectly aligned with the primary axis. This kind of unintended direction component in the movement of the primary resonator is referred to as quadrature error.
Quadrature error may be compensated in example by physically trimming the resonator element. This type of compensation is often used for piezoelectric gyroscopes. Compensation may also comprise electrical tuning, actively tuning electrical spring forces in parallel plate capacitors. This type of compensation is often used in silicon gyroscopes. Further, the effects of quadrature error may be alleviated through design of the gyroscope, particularly by design of the springs. U.S. Pat. No. 8,104,343 presents use of compensation grooves etched into a spring structure to alleviate unwanted coupling caused by a non-ideality. Different compensation methods may be used separately, or in a combination suitable for the specific implementation.
US patent application publication US2006/0156814 presents a micromechanical sensor of angular velocity which has two pairs of springs each arranged for allowing one of the two perpendicular oscillation motions, and a frame structure connecting said pairs of springs.